Question

Which of the following is not a thermal decomposition reaction?

• A. $2H_{2}O\to 2H_2+O_2$
• B. $2FeS{O}_4\to F{e}_2{O}_3+S{O}_2+S{O}_3$
• C. $ZnC{O}_3\to ZnO+C{O}_2$
• D. $2KCl{O}_3\to 2KCl+3O_2$

Answer 1

The correct option is A

$2H_{2}O\to 2H_2+O_2$

Explanation for correct option:

(A)

• The reaction in which a single reactant dissociates into two or more products is called a decomposition reaction. When the decomposition occurs in the presence of heat then it is termed as a thermal decomposition reaction.
• The given reaction is as follows:
• $2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\stackrel{\mathrm{Electricity}}{\to }2{\mathrm{H}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)$
• Since water $\left({\mathrm{H}}_2\mathrm{O}\right)$ decomposes to form hydrogen molecule $\left({\mathrm{H}}_2\right)$ and oxygen molecule $\left({\mathrm{O}}_2\right)$ in the presence of electricity but not heat, thus, it is classified as an electrolysis process not a thermal decomposition reaction.

Explanation for incorrect options:

(B) The given reaction is as follows:

$\underset{\mathrm{Ferrous}\mathrm{sulfate}}{2{\mathrm{FeSO}}_4\left(\mathrm{s}\right)}\stackrel{∆}{\to }\underset{\mathrm{Iron}\left(\mathrm{III}\right)\mathrm{oxide}}{{\mathrm{Fe}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)}+\underset{\mathrm{Sulfur}\mathrm{dioxide}}{{\mathrm{SO}}_2\left(\mathrm{g}\right)}+\underset{\mathrm{Sulfur}\mathrm{trioxide}}{{\mathrm{SO}}_3\left(\mathrm{g}\right)}$

Decomposition of iron (II) sulfate or ferrous sulfate $\left({\mathrm{FeSO}}_4\right)$ occurs in the presence of heat. Thus, it is an example of a thermal decomposition reaction.

(C) The given reaction is as follows:

$\underset{\mathrm{Zinc}\mathrm{carbonate}}{{\mathrm{ZnCO}}_3\left(\mathrm{s}\right)}\stackrel{∆}{\to }\underset{\mathrm{Zinc}\mathrm{oxide}}{\mathrm{ZnO}\left(\mathrm{s}\right)}+\underset{\mathrm{Carbon}\mathrm{dioxide}}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}$

Zinc carbonate $\left({\mathrm{ZnCO}}_3\right)$ undergoes decomposition in the presence of heat. Thus, it is an example of a thermal decomposition reaction.

(D) The given reaction is as follows:

$\underset{\mathrm{Potassium}\mathrm{chlorate}}{2{\mathrm{KClO}}_3\left(\mathrm{s}\right)}\stackrel{∆}{\to }\underset{\mathrm{Potassium}\mathrm{chloride}}{2\mathrm{KCl}\left(\mathrm{s}\right)}+\underset{\mathrm{Oxygen}\mathrm{gas}}{3{\mathrm{O}}_2\left(\mathrm{g}\right)}$

Decomposition of potassium chlorate $\left({\mathrm{KClO}}_3\right)$ to produce potassium chloride $\left(\mathrm{KCl}\right)$ and oxygen molecule $\left({\mathrm{O}}_2\right)$ occurs in the presence of heat. Thus, it is an example of a thermal decomposition reaction.

Hence, option (A) is correct.